The invention is directed to a broadband amplifier having an upper limit frequency of several GHz.
Broadband amplifiers of this type are known as distributed amplifiers (Texas Instruments TGA8334-SCC 2-TO20-GHz Power Amplifier, data sheet 1990 or Halladay, Pavio, Crabill "A 1-20 GHz dual gate distributed power amplifier" 1987, IEEE GaAs IC Symposium, page 219). It is also known to provide an input circuit between the gate terminal of each of the transistors that are connected in parallel and the input line, the input circuit being a parallel circuit of a capacitor and an ohmic resistor. Although the power distribution onto the control inputs of the transistors connected in parallel is thereby somewhat improved, an actually uniform distribution for all frequencies cannot be achieved with this known circuit. In the known circuit, the capacitors are dimensioned such that the capacitances increase continuously from the beginning of the input line at the successive transistors up to the end of the input line, so that the transistors lying farther back receive a comparatively higher frequency part of the input power that is already considerably reduced by losses given the high frequencies. The values of the resistors connected in parallel to the capacitors, however, can only be varied within a narrow range in this known circuit. On the one hand, the resistors must be so small that the gate currents of the transistors do not lead to different operating points; on the other hand, they must be large enough so that they do not cancel the effect of the capacitances at high frequencies.